This invention relates generally to illumination arrangements for instrument dials and more particularly to an improved illumination system for a wristwatch dial.
It is known to provide instruments such as wristwatches with a light source for illuminating the dial. The wristwatch dial may be an analog wristwatch dial having indicia or markers used in connection with hands of the watch, or the dial may comprise an electro-optic display of actuated indicia, such as a liquid crystal display. One objective in a dial illumination system has been to achieve a constant and uniform lighting for the dial. Uniform background lighting may be achieved by using a distributed uniform light source, such as an electroluminescent light source. However, in order to reduce the cost of an illumination system for a dial, concentrated light sources, such as a single incandescent bulb or a light emitting diode (LED) have also been considered. One problem with using a concentrated light source is to transfer and diffuse the light from a point source of bright light to a uniformly distributed light source for illuminating the dial. The prior art contains many examples of efforts to effect the transformation.
In addition to illumination, light may be used to achieve an aesthetic graphic presentation. In this case it must be possible to locate areas of greater or lesser brightness or different colors at different locations on the dial. Traditional constructions using edge lighting around the outside periphery of a light guide are ineffective in directing the light to the desired location.
Constructions are known for using a light guide behind a light transmissive dial, and to inject light into the light guide. The following U.S. Patents are representative of prior art constructions for illuminating dials from a concentrated light source using a light guide.
U.S. Pat. No. INVENTOR ISSUE DATE TITLE 3,748,456 Brien July 24, 1973 Illuminated Instrument Dials 2,140,972 Rylsky Dec. 20, 1938 Means far Illuminating Dials of Instruments 2,188,821 Rylsky Jan. 30, 1940 Compass 2,480,393 Bossert et al. Aug. 30, 1949 Dial Light 3,043,038 Marble July 10, 1962 Illuminated Dial Face 3,561,145 Shotwell Feb. 9, 1971 Light Distributing Lens System
The aforementioned Marble U.S. Pat. No. 3,043,038 and Shotwell U.S. Pat. No. 3,561,145 employ light guides bounded is by substantially planar surfaces and attempt to distribute the light rays which are undergoing multiple internal reflections which are introduced from a concentrated source at the peripheral edge of the light guide. Such a construction is deficient for several reasons. For example, in the '038 patent identified above, the light entering the lens must do so at an edge and therefore, the distribution of light along the entire lens surface is less than uniform. Secondly, and as conceded in the '038 patent, there is a tremendously non-uniform illumination in the lens nearest the lamp. Hence, the '038 patent employs an opaque member that causes less than the entire dial surface to illuminate. Although this is considered by the '038 patent to be a "novel effect," it is clear that it is less than desirable in a watch where it is desirable to illuminate the entire dial. Lastly positioning the lamp adjacent the lens is permissible in a device where space is not a priority. In watches, space is a valuable commodity that must be considered at all times.
U.S. Pat. No. 5,667,289 issued Sep. 16, 1997 to Akahane et al. describes a background lighting apparatus for a liquid crystal display, in which a nonuniform pattern of light dispersing reflective layers are applied to a light guide illuminated from the edges of the light guide. The reflective layers are arranged to increase in size as the distance from the lighted edge become greater.
It is particularly difficult to effectively and uniformly conduct light from a concentrated source to a light guide in a wristwatch disposed beneath the watch dial. Space is at a premium and the previously known constructions have not been satisfactory.
Accordingly, an improved illumination arrangement for a watch dial that overcomes the drawbacks found in the prior art while at the same time provides for a desirable and uniform distribution of illumination, is desired.